? ? ? ?在開發過程中,雖然感覺優化sql語句很重要,但是往往更加重視的是功能實現,為了使自己以后寫mysql語句效率更高,有必要對mysql優化做一個小小歸納。
步驟一、通過show status 命令了解各種sql執行的效率
show [session|gobal] status
session級別表示統計當前連接的結果。
global級別表示統計自數據上次啟動至今的結果。
如果不寫級別,默認的是session級別
eg:SHOW GLOBAL STATUS;
Variable_name | Value |
Aborted_clients | 6 |
Aborted_connects | 0 |
Binlog_cache_disk_use | 0 |
Binlog_cache_use | 0 |
Binlog_stmt_cache_disk_use | 0 |
Binlog_stmt_cache_use | 0 |
Bytes_received | 95645 |
Bytes_sent | 1285066 |
Com_admin_commands | 0 |
Com_assign_to_keycache | 0 |
Com_alter_db | 0 |
Com_alter_db_upgrade | 0 |
Com_alter_event | 0 |
Com_alter_function | 0 |
Com_alter_procedure | 0 |
Com_alter_server | 0 |
Com_alter_table | 6 |
Com_alter_tablespace | 0 |
Com_alter_user | 0 |
Com_analyze | 0 |
Com_begin | 0 |
Com_binlog | 0 |
Com_call_procedure | 0 |
Com_change_db | 8 |
Com_change_master | 0 |
Com_check | 0 |
Com_checksum | 0 |
Com_commit | 0 |
Com_create_db | 0 |
Com_create_event | 0 |
Com_create_function | 0 |
Com_create_index | 0 |
Com_create_procedure | 0 |
Com_create_server | 0 |
Com_create_table | 5 |
Com_create_trigger | 0 |
Com_create_udf | 0 |
Com_create_user | 0 |
Com_create_view | 6 |
Com_dealloc_sql | 0 |
Com_delete | 2 |
Com_delete_multi | 0 |
Com_do | 0 |
Com_drop_db | 0 |
Com_drop_event | 0 |
Com_drop_function | 0 |
Com_drop_index | 0 |
Com_drop_procedure | 0 |
Com_drop_server | 0 |
Com_drop_table | 0 |
Com_drop_trigger | 0 |
Com_drop_user | 0 |
Com_drop_view | 1 |
Com_empty_query | 2 |
Com_execute_sql | 0 |
Com_flush | 0 |
Com_get_diagnostics | 0 |
Com_grant | 0 |
Com_ha_close | 0 |
Com_ha_open | 0 |
Com_ha_read | 0 |
Com_help | 0 |
Com_insert | 15 |
Com_insert_select | 0 |
Com_install_plugin | 0 |
Com_kill | 0 |
Com_load | 0 |
Com_lock_tables | 0 |
Com_optimize | 0 |
Com_preload_keys | 0 |
Com_prepare_sql | 0 |
Com_purge | 0 |
Com_purge_before_date | 0 |
Com_release_savepoint | 0 |
Com_rename_table | 0 |
Com_rename_user | 0 |
Com_repair | 0 |
Com_replace | 0 |
Com_replace_select | 0 |
Com_reset | 0 |
Com_resignal | 0 |
Com_revoke | 0 |
Com_revoke_all | 0 |
Com_rollback | 0 |
Com_rollback_to_savepoint | 0 |
Com_savepoint | 0 |
Com_select | 414 |
Com_set_option | 525 |
Com_signal | 0 |
Com_show_binlog_events | 0 |
Com_show_binlogs | 0 |
Com_show_charsets | 0 |
Com_show_collations | 0 |
Com_show_create_db | 0 |
Com_show_create_event | 0 |
Com_show_create_func | 0 |
Com_show_create_proc | 0 |
Com_show_create_table | 260 |
Com_show_create_trigger | 0 |
Com_show_databases | 8 |
Com_show_engine_logs | 0 |
Com_show_engine_mutex | 0 |
Com_show_engine_status | 0 |
Com_show_events | 0 |
Com_show_errors | 0 |
Com_show_fields | 102 |
Com_show_function_code | 0 |
Com_show_function_status | 0 |
Com_show_grants | 0 |
Com_show_keys | 86 |
Com_show_master_status | 0 |
Com_show_open_tables | 0 |
Com_show_plugins | 0 |
Com_show_privileges | 0 |
Com_show_procedure_code | 0 |
Com_show_procedure_status | 0 |
Com_show_processlist | 1 |
Com_show_profile | 0 |
Com_show_profiles | 115 |
Com_show_relaylog_events | 0 |
Com_show_slave_hosts | 0 |
Com_show_slave_status | 0 |
Com_show_status | 247 |
Com_show_storage_engines | 0 |
Com_show_table_status | 1 |
Com_show_tables | 14 |
Com_show_triggers | 5 |
Com_show_variables | 5 |
Com_show_warnings | 0 |
Com_slave_start | 0 |
Com_slave_stop | 0 |
Com_stmt_close | 0 |
Com_stmt_execute | 0 |
Com_stmt_fetch | 0 |
Com_stmt_prepare | 0 |
Com_stmt_reprepare | 0 |
Com_stmt_reset | 0 |
Com_stmt_send_long_data | 0 |
Com_truncate | 0 |
Com_uninstall_plugin | 0 |
Com_unlock_tables | 0 |
Com_update | 27 |
Com_update_multi | 0 |
Com_xa_commit | 0 |
Com_xa_end | 0 |
Com_xa_prepare | 0 |
Com_xa_recover | 0 |
Com_xa_rollback | 0 |
Com_xa_start | 0 |
Compression | ON |
Connection_errors_accept | 0 |
Connection_errors_internal | 0 |
Connection_errors_max_connections | 0 |
Connection_errors_peer_address | 0 |
Connection_errors_select | 0 |
Connection_errors_tcpwrap | 0 |
Connections | 10 |
Created_tmp_disk_tables | 128 |
Created_tmp_files | 5 |
Created_tmp_tables | 910 |
Delayed_errors | 0 |
Delayed_insert_threads | 0 |
Delayed_writes | 0 |
Flush_commands | 1 |
Handler_commit | 108 |
Handler_delete | 2 |
Handler_discover | 0 |
Handler_external_lock | 782 |
Handler_mrr_init | 0 |
Handler_prepare | 0 |
Handler_read_first | 73 |
Handler_read_key | 2109 |
Handler_read_last | 0 |
Handler_read_next | 42 |
Handler_read_prev | 0 |
Handler_read_rnd | 1882 |
Handler_read_rnd_next | 94791 |
Handler_rollback | 0 |
Handler_savepoint | 0 |
Handler_savepoint_rollback | 0 |
Handler_update | 195 |
Handler_write | 93316 |
Innodb_buffer_pool_dump_status | not started |
Innodb_buffer_pool_load_status | not started |
Innodb_buffer_pool_pages_data | 397 |
Innodb_buffer_pool_bytes_data | 6504448 |
Innodb_buffer_pool_pages_dirty | 0 |
Innodb_buffer_pool_bytes_dirty | 0 |
Innodb_buffer_pool_pages_flushed | 193 |
Innodb_buffer_pool_pages_free | 7795 |
Innodb_buffer_pool_pages_misc | 0 |
Innodb_buffer_pool_pages_total | 8192 |
Innodb_buffer_pool_read_ahead_rnd | 0 |
Innodb_buffer_pool_read_ahead | 0 |
Innodb_buffer_pool_read_ahead_evicted | 0 |
Innodb_buffer_pool_read_requests | 4642 |
Innodb_buffer_pool_reads | 364 |
Innodb_buffer_pool_wait_free | 0 |
Innodb_buffer_pool_write_requests | 872 |
Innodb_data_fsyncs | 129 |
Innodb_data_pending_fsyncs | 0 |
Innodb_data_pending_reads | 0 |
Innodb_data_pending_writes | 0 |
Innodb_data_read | 6033408 |
Innodb_data_reads | 402 |
Innodb_data_writes | 281 |
Innodb_data_written | 6534656 |
Innodb_dblwr_pages_written | 193 |
Innodb_dblwr_writes | 14 |
Innodb_have_atomic_builtins | ON |
Innodb_log_waits | 0 |
Innodb_log_write_requests | 574 |
Innodb_log_writes | 46 |
Innodb_os_log_fsyncs | 61 |
Innodb_os_log_pending_fsyncs | 0 |
Innodb_os_log_pending_writes | 0 |
Innodb_os_log_written | 202752 |
Innodb_page_size | 16384 |
Innodb_pages_created | 34 |
Innodb_pages_read | 363 |
Innodb_pages_written | 193 |
Innodb_row_lock_current_waits | 0 |
Innodb_row_lock_time | 0 |
Innodb_row_lock_time_avg | 0 |
Innodb_row_lock_time_max | 0 |
Innodb_row_lock_waits | 0 |
Innodb_rows_deleted | 0 |
Innodb_rows_inserted | 3 |
Innodb_rows_read | 406 |
Innodb_rows_updated | 2 |
Innodb_num_open_files | 32 |
Innodb_truncated_status_writes | 0 |
Innodb_available_undo_logs | 128 |
Key_blocks_not_flushed | 0 |
Key_blocks_unused | 14344 |
Key_blocks_used | 3 |
Key_read_requests | 381 |
Key_reads | 1 |
Key_write_requests | 117 |
Key_writes | 50 |
Last_query_cost | 0.000000 |
Last_query_partial_plans | 0 |
Max_used_connections | 3 |
Not_flushed_delayed_rows | 0 |
Open_files | 70 |
Open_streams | 0 |
Open_table_definitions | 120 |
Open_tables | 117 |
Opened_files | 1042 |
Opened_table_definitions | 144 |
Opened_tables | 147 |
Performance_schema_accounts_lost | 0 |
Performance_schema_cond_classes_lost | 0 |
Performance_schema_cond_instances_lost | 0 |
Performance_schema_digest_lost | 0 |
Performance_schema_file_classes_lost | 0 |
Performance_schema_file_handles_lost | 0 |
Performance_schema_file_instances_lost | 0 |
Performance_schema_hosts_lost | 0 |
Performance_schema_locker_lost | 0 |
Performance_schema_mutex_classes_lost | 0 |
Performance_schema_mutex_instances_lost | 0 |
Performance_schema_rwlock_classes_lost | 0 |
Performance_schema_rwlock_instances_lost | 0 |
Performance_schema_session_connect_attrs_lost | 0 |
Performance_schema_socket_classes_lost | 0 |
Performance_schema_socket_instances_lost | 0 |
Performance_schema_stage_classes_lost | 0 |
Performance_schema_statement_classes_lost | 0 |
Performance_schema_table_handles_lost | 0 |
Performance_schema_table_instances_lost | 0 |
Performance_schema_thread_classes_lost | 0 |
Performance_schema_thread_instances_lost | 0 |
Performance_schema_users_lost | 0 |
Prepared_stmt_count | 0 |
Qcache_free_blocks | 1 |
Qcache_free_memory | 1039896 |
Qcache_hits | 0 |
Qcache_inserts | 0 |
Qcache_lowmem_prunes | 0 |
Qcache_not_cached | 404 |
Qcache_queries_in_cache | 0 |
Qcache_total_blocks | 1 |
Queries | 1888 |
Questions | 1887 |
Select_full_join | 1 |
Select_full_range_join | 0 |
Select_range | 23 |
Select_range_check | 0 |
Select_scan | 727 |
Slave_heartbeat_period | 0.000 |
Slave_last_heartbeat | ? |
Slave_open_temp_tables | 0 |
Slave_received_heartbeats | 0 |
Slave_retried_transactions | 0 |
Slave_running | OFF |
Slow_launch_threads | 0 |
Slow_queries | 0 |
Sort_merge_passes | 0 |
Sort_range | 0 |
Sort_rows | 1964 |
Sort_scan | 151 |
Ssl_accept_renegotiates | 0 |
Ssl_accepts | 0 |
Ssl_callback_cache_hits | 0 |
Ssl_cipher | ? |
Ssl_cipher_list | ? |
Ssl_client_connects | 0 |
Ssl_connect_renegotiates | 0 |
Ssl_ctx_verify_depth | 0 |
Ssl_ctx_verify_mode | 0 |
Ssl_default_timeout | 0 |
Ssl_finished_accepts | 0 |
Ssl_finished_connects | 0 |
Ssl_server_not_after | ? |
Ssl_server_not_before | ? |
Ssl_session_cache_hits | 0 |
Ssl_session_cache_misses | 0 |
Ssl_session_cache_mode | NONE |
Ssl_session_cache_overflows | 0 |
Ssl_session_cache_size | 0 |
Ssl_session_cache_timeouts | 0 |
Ssl_sessions_reused | 0 |
Ssl_used_session_cache_entries | 0 |
Ssl_verify_depth | 0 |
Ssl_verify_mode | 0 |
Ssl_version | ? |
Table_locks_immediate | 386 |
Table_locks_waited | 0 |
Table_open_cache_hits | 656 |
Table_open_cache_misses | 130 |
Table_open_cache_overflows | 0 |
Tc_log_max_pages_used | 0 |
Tc_log_page_size | 0 |
Tc_log_page_waits | 0 |
Threads_cached | 1 |
Threads_connected | 2 |
Threads_created | 3 |
Threads_running | 1 |
Uptime | 286258 |
Uptime_since_flush_status | 286258 |
主要參數描述
Connections:視圖連接mysql服務器的次數
Uptime:服務器工作時間
Slow_queries:慢查詢的次數
Com_xxx表示每個xxx語句執行的次數
Com_select ?執行select次數
Com_insert ?執行insert次數,批量插入時候,只累加1次
Com_update 執行Update操作次數
Com_delete 執行刪除操作次數
innodb_rows_xxx類型的參數只對innodb存儲引擎有效
解析:通過上面一些參數,可以容易了解當前數據庫的應用時插入更新為主還是查詢為主,以及執行比例。對應更新操作的計數,是對執行次數的計數,不論提交還是回滾都會累加。
步驟二:定位執行效率較低的sql語句
通過慢查詢日志定位哪些是執行效率的sql語句。用–log-slow-queries[=file_name]選項啟動,mysqld寫一個包含所有執行超過long_query_time秒的sql語句的日志文件。關于如何定位慢查詢可以點解這里(http://www.php.cn/)
步驟三:通過EXPLAIN分析低效率SQL的執行計劃
在步驟二中,我們可以查詢到低效率的sql語句,在此我們通過explain或desc可以獲取mysql執行select信息。
eg:
EXPLAIN? SELECT? ??t0.*? FROM ??t3?AS?t0 ??LEFT?JOIN?`t2`?AS?t1? ????ON?t0.`id1`?=?t1.`id1`? WHERE?t0.id1?=?5?;
執行結果如下:
解析:select_type:表示select類型。常見的取值有SIMPLE(簡單表,即不使用連接或者子查詢)、PRIMARY(主查詢,即外層的查詢)、UNION(union中的第二個或者后面的查詢語句)、SUBQUERY(子查詢中的第一個SELECT)等。
talbe:輸出結果集的表。
type:表的連接類型。性能由高到底:system(表中僅有一行)、const(表中最多有一個匹配行)、eq_ref、ref、ref_null、index_merge、unique_subquery、index_subquery、range、idnex等
possible_keys:查詢時,可能使用的索引
key:實際使用的索引
key_len:索引字段的長度
rows:掃描行的數量
Extra:執行情況的說明和描述
步驟四:確定問題并采取相應的優化措施
經過上面的步驟,可以確定問題出現的原因,此時我們可以根據情況,采取相應的措施。常見的措施有1.建立相應的索引2.優化sql語句3.分表等。
備注:如果索引正在工作,handler_read_key的值將很高,這個值代表了一個行被索引值讀的次數,很低的值表名增加索引得到的性能改善不高,因為索引并不經常使用。handler_read_rnd_next值高則意味著查詢運行低效,并且應該建立索引補救。如果正在進行大量的表掃描,handler_read_rnd_next值較高,則通常說明表索引不正確或寫入的查詢沒有利用索引,如下圖。
我們應當定期分析表盒檢查表
檢查表使用如下命令(檢查t3表)
ANALYZE TABLE t3;
CHECK TABLE t3;
定期優化表使用命令如下
optimize table 表名
常見語句優化
1.優化Insert語句
(1)如果從同一個客戶端插入數據,盡力使用多個字表的insert語句和多行插入,減少單行插入,這種方式大大減少客戶端與數據庫直接的連接、關閉等消耗。eg:
INSERT INTO t3 VALUES(1,2),(8,5),(6,5),(4,3)
(2)如果從不同的客戶插入很多行,能通過使用inset delayed語句得到更高的速度。
(3)如果進行批量插入,可以增加bulk_insert_buffer_size變量方法,提高速度。
2.優化group by語句
默認情況下使用group by col1,col2….會對查詢進行相應的排序,如果用戶想要避免排序結果的消耗,可以指定order by null 禁止排序。通過查詢結果中extra字段可以看出:
3.優化order by語句
在某些情況中,mysql可以使用一個索引來滿足order by子句,而不需要額外的排序,where條件和order by使用相同的索引。
4.優化含有or語句
對于含有or的查詢子句,如果要使用索引,則or之間的每個條件列必須用到索引;否則,應該考慮添加索引。
5.使用sql提示
以上就是MySQL優化的步驟詳解的內容,更多相關內容請關注PHP中文網(www.php.cn)!